1. Field of the Invention
The present invention relates to a method of separating and purifying nuclear fuel substances using a difference in fluorination volatility behavior of uranium, plutonium and other elements in a spent oxide fuel to reprocess the spent oxide fuel.
2. Description of the Related Art
A fluoride volatility process is one of methods for dry reprocessing of a spent fuel, in which nuclear fuel substances such as uranium and plutonium and various kinds of nuclear fission products are separated and recovered using a difference in volatility behavior when they are fluorinated. Techniques for applying the fluoride volatility process to a reprocessing process have been developed in the U.S. and other various countries since 1950s. However, each of those techniques has problem in higher fluorination and purification of plutonium. None of these techniques has reached a practical phase, and there has been no progress since 1970s.
In Japan, Japan Atomic Energy Research Institute has carried out the fluoride volatility process, and many advantages have been found, but development of the technique has been ended without establishing a plutonium purification step. In the fluoride volatility process carried out in that institute, uranium and plutonium are separated by two-stage fluorination using a fluidized bed furnace as a reactor with the temperature and fluorine concentration being changed. For example, in the first stage, uranium is fluorinated with an F2 concentration of 20% at the operating temperature of 330° C., and in the second stage, plutonium is fluorinated with an F2 concentration of 100% at the operating temperature of 330 to 550° C.
However, there is a disadvantage that it takes much time to convert uranium into uranium hexafluoride (UF6) because the reaction temperature in “fluorination of uranium” of the first stage is low, and that plutonium easily forms the most stable plutonium tetrafluoride (PuF4). Further, in “fluorination of plutonium” of the second stage, there is a disadvantage that plutonium is hard to be converted into plutonium hexafluoride (PuF6) (the conversion ratio or conversion rate decreases) from the point of view of thermodynamics and reaction temperature because plutonium forms into PuF4 of an intermediate fluoride in the first stage, and the fluorine concentration is so high that incomplete fluidization easily occurs.
Thus, a reprocessing process according to a fluoride volatility process using a flame furnace as a reactor has been proposed (see, for example, Japanese Patent Laid-Open Specification No. 2001-153991). Unlike the fluidized bed furnace, the flame furnace is a reactor operating under conditions of high temperature and high fluorine gas atmosphere.
If plutonium is converted under the conditions, a direct fluorination reaction of PuO2+3F2 (or 6F)→PuF6+O2 occurs, and therefore PuF4 is never produced as an intermediate fluoride. In addition, because the fluorination temperature and fluorine concentration are high, decomposition reaction of PuF6 is hard to proceed. However, since a corrosive gas at high temperatures and in high concentrations is required for the reaction, and conversion conditions are severe, the reactor is easily corroded and deteriorated, thus causing a problem in terms of materials. In addition, there are disadvantages that temperature adjustment for conversion conditions of a target substance is impossible, a large amount of expensive fluorine gas is used, and so on.